Solid electrolyte galvanic oxygen sensors essentially include an oxygen ion-conductive ceramic body with porous electrodes on opposite faces of the body. One electrode is exposed to a reference source of oxygen. The other electrode is exposed to a source whose oxygen content is to be measured. A difference in oxygen partial pressure at the electrodes results in a corresponding electrode potential difference, providing a sensor output voltage.
The output voltage of such sensors can be used to measure oxygen or unburned combustibles in combination gases produced by an internal combustion engine. This voltage can be used in monitoring and controlling the combustion process, as disclosed in U.S. Pat. Nos. 3,616,274 Eddy, 3,844,920 Burgett et al and U.S. Ser. No. 787,900 Howarth, filed Apr. 15, 1977.
The sensor solid electrolyte must be at an elevated temperature to obtain an appreciable output voltage. Moreover, sensor output voltage varies with changes in temperature, especially at lower operating temperatures. Combustion gases can be used to heat the sensor to operating temperatures but such gases vary widely in temperature, particularly when from an internal combustion engine. It may be desirable in some applications to include a heater in the sensor. For automotive applications, the heated sensor should be particularly rugged and reliable. In addition, for higher reliability and lower cost, the heated sensor should be simple and readily manufacturable. United States Patent application Ser. No. 892,644, entitled "Heated Solid Electrolyte Oxygen Sensor" filed Apr. 3, 1978, in the name of M. P. Murphy, discloses forming a subassembly of the heater and the sensor reference, i.e. air, electrode terminal. In the subassembly, the heater is prealigned on the electrode terminal. The electrode terminal contacts the reference electrode in a predetermined alignment. When the reference electrode terminal-heater subassembly is assembled with its solid electrolyte, the heater is thus also inherently aligned with the solid electrolyte. The heater-electrode terminal subassembly approach is particularly useful for adding a heater to the oxygen sensor disclosed in U.S. Pat. No. 3,844,920 Burgett et al.
An elongated heater is insulatingly supported on the reference electrode terminal in a particularly unique manner that is described and claimed in United States patent application Ser. No. 892,642, entitled "Solid Electrolyte Oxygen Sensor with Electrically Isolated Heater", filed Apr. 3, 1978, in the names of M. P. Murphy and R. D. Willis. In this latter invention the elongated heater is supported by a vitrified bond within at least one ceramic sleeve that is, in turn, supported by a vitrified bond within the tubular portion of the reference electrode terminal. Means coacting with terminal and housing flanges hold the electrolyte member, heater-terminal subassembly and housing in a fixed predetermined concentric relationship. Precise alignment is assured, even though the components are readily assemblable. The vitrified bond can provide a simple, rugged, reliable and readily manufacturable subassembly for a heated oxygen sensor in which the heater is electrically isolated from the sensor electrodes.
On the other hand, affixing the elongated heater to the ceramic sleeve and electrode terminal by a vitrified bond requires a furnace treatment of the subassembly. Furnace treatments can be costly in that they are slow and require extensive plant floor space. Energy consumption can be expensive too. Also, glass bonding may not be as rugged or reliable as may be desired for severe circumstances. I have found how the heater can be insulatingly supported on the electrode terminal without need for a fused glass bond. I have found how to mechanically support the heater on the electrode terminal, and still keep the two electrically and thermally isolated. In a preferred sensor and method of making it, the elongated heater has a swaged cylindrical metal sheath. The sheath has an enlargement intermediate its ends that is integrally formed while the sheath is being swaged. The enlargement provides a mechanical interlock with the ceramic sleeves for mechanically affixing the heater to the electrode terminal by a simple, rapid and rugged crimping operation.